Question: In geometry class, the girl to boy ratio is $3$ to $7$. If there are a total of $30$ students, how many girls are there?
Explanation: A ratio of $3$ girls to $7$ boys means that a set of $10$ students will have $3$ girls and $7$ boys. A class of $30$ students has $3$ sets of $10$ students. Because we know that there are $3$ girls in each set of $10$ students, the class must have $3$ groups of $3$ girls each. There is a total of $9$ girls in geometry class.